Various Methods of Moulding.
COM MOM rwo-pA/ir BOX Ifytf' 2,-
these'are sketched at Fig. 2. The boxes are light castings, ribbed
across as shewn, allowing space for the escape of gas from the
molten metal. (See Appendix //.,/. 781.)
Sand used in moulding is of two kinds, green sand and loam
^ Green sand is obtained from the chalk or coal measures, that
of the London basin being among the best. Green sand should
contain a large percentage of silica to give porosity, together with
a very little magnesia and alumina for binding purpose's. The
lining of Bessemer converters has about 85 percent, of silica in its
composition, while many moulders prefer to have as much as 93 or
96 per cent, of silica, leaving only 4 or 7 per cent, of other substances.
The sand should not burn on setting, or it will stick too much when
wetted for use again, and, while cohesion is necessary, it should at
the same time be porous enough to allow for the passage of air,
though not so much as to permit of any molten metal entering it.
v Loam is a mixture of clay (ferruginous or calcareous*) with a
considerable amount of rock sand (abraded rock). It is ground
in a mortar-mill and mixed with powdered charcoal, horse dung,
cow hair, chaff. &c., to give it binding power and porosity.
Besides the above, Cores require a mixture of rock sand and
sea sand (the latter for porosity), and Parting Sand, for the use
Implied by its name, consists of finely powdered blast-furnace
cinder, brickdust, or fine dust from castings; ail perfectly dry.
Moulding is practised by three different methods: Green
Sand, Dry Sand, and Loam Moulding.
* With iron or lime respectively. •